Method and apparatus for electrical precipitation



H. J. WHITE 2,249,801

METHOD AND APPARATUS FOR ELECTRICAL PRECiPITATION July 22, 1941.

Filed Jan. 22, 1940 4 Sheets-Sheet l INVENTOR ARRY rI WHITE,

BY v I ATTORNEY y 1941- H. J. WHITE 2,249,801

METHOD AND APPARATUS FOR ELECTRICAL PRECIPITATION Filed Jan. 22, 1940 4Sheets-Sheet 2 & w

INVENTOR HARRY a: WH/TE,

ATTORNEY July 22, 1941. H,- WHITE 2,249,801

METHOD AND APPARATUS FOR ELECTRICAL PRECIPITATION Filed Jan. 22, 1940 4Sheets-Sheet s :5 31 19 Z0 gag, I 15 I L I I al 1 A I l, l\ I 30 28a 31-INVENTOR H4 REY r2 WH/ TE,

' ATTORNEY July 22, 1941. H. J WHITE 2,249,801

METHOD AND APPARATUS FOR ELECTRICAL PRECIPITATION Filed Jan. 22, 1940 4Sheets-Sheet 4 INVENTOE ff/LQRY 11 710-1175,

ATTORNEY Patented July 22, 1941 UNITED STATES PATENT OFFICE ME'rnonaNnAPPARATUS ron ELECTRICAL rnuorrrrarron Harry J. White, Los Angeles, cum,assignor to Research Corporation, New York, N. Y., a corporation of NewYork Application January 2:, 1940, Serial No. 315,111

Claims.

The present invention is generally concerned with the art ofelectrically precipitating suspended particles from a stream of gas; andit is In its basic form, a precipitator of this type has a first fieldmaintained between two electrodes of which one is a discharge electrodeand the other is a non-discharging electrode, and a second fieldmaintained between two' substantially non-discharging electrodes.

In the following description and appended claims, the term dischargeelectrode will be understood to designate an electrode that facilitatescorona discharge therefrom, because it has a configuration thatestablishes a sufllciently high potential gradient at or near itssurface to create corona discharge before there is a disruptivedischarge or spark-over. For this purpose, the discharge electrodeusually takes the form of a member of small surface area, such asa-small diameter wire or a rod provided with sharp edges or points,whereby there may be created in the immediate vicinity thereof asufllciently high electric field intensity to cause ionization andcorona discharge. The term non-discharging electrode will be understoodto designate an electrode that minimizes or prevents corona dischargetherefrom because it has a configuration that establishes a sumcientlylow field concentration at or near the surface to suppress coronadischarge at elevated potentials lower than the voltage required fordisruptive discharge or spark-over. For this purpose, a non-dischargingelectrode is usually one of extended surface area, substantially freefrom sharp comers or other parts of sharp surface curvature at allportions which are located within the electric field,so as tosubstantially avoid ionization or corona discharge at that electrode.

The construction and advantages of an electrical precipitator of thetwo-stage or separated field type are discussed in Patent No. 1,343,285granted June 15, 1920, to W. A. Schmidt and also in my co-pendingapplication Serial No. 248,348 filed December 29, 1338, for Method andapparatus for electrical precipitation." Briefly, these advantages areeconomy of installation and operation; advantageous inherent operatingcharacteristics with certain types of dust; a much reduced tendency tothe detrimental phenomenon of back-corona" in the collecting section;and possibility of maintaining a. relatively higher potential gradientbetween the precipitating electrodes in the collecting section, thusbringing about very effective collection.

The phenomenon of back-corona arises when dust of relativelynon-conducting characteristics is being collected in a precipitatora Forthe purposes or this disclosure, back-corona may be considered to becorona discharge at or in the immediate vicinity of a non-dischargingelectrode that is opposing a discharge electrode. The

20 back-corona discharge is consequently opposite in electrical sign tnthe main discharge and therefore produces ions of an opposite sign tothe main supply of ions. Since a discharge electrode usually producesnegative ions, the back 85' discharge ordinarily is thought of asproducing positive ions. The mechanics of the occurrence of back-coronaare not entirely understood, but apparently are as follows.

Particles of non-conducting dust collect upon the surface of anon-discharging electrode opposing a discharge electrode and eventuallythe collection is suillcient that, even though the layer is very thin,it is substantially continuous and has the effect of insulating theelectrode from the space around it. As a result of the presence of thisinsulating layer, there is an alteration in the potential and fielddistribution around the electrode in such a way that a relatively largerproportion of the total potential drop between the electrodes occursacross the dust layer. The immediate result" is a decrease in voltageacross the gas and a decrease in corona current, with a consequentreduction in efflciency of the pre- 5 cipitator. Eventually the voltagedrop across the dust layer becomes sufiiclently great that thelayerbreaks down locally, a small hole being formed in' the layer intowhich the gas flows. In the vicinity of this hole, the gas becomes agood conductor and there is a concentration of the field andthedischarge at this hole. The result is either a spark-over between theelectrodes or back-corona discharge from the normally nondischargingelectrode. Whether spark-over or back-corona appears depends uponvarious factors, and sometimes the back-corona gives way to aspark-over.

Several undesirable conditions that reduce the precipitator eiilciencyresult from back-corona. The spark-over voltage is greatly lowered andit is necessary that the applied voltage be reduced in order to maintaincorona discharge at the discharge electrode. Also, there isa'concentration of discharge at the points of local rupture in the dustlayer with the result that there may be large sections of the spacethrough which the gas stream flows that have relatively few ions.

This means that there is a lower average charge on the dust particles,or in other words that the charging ability of the preclpitator isdecreased. Probably the most important result of back-corona is thatpositive ions produced at the points of back-corona enter the gasstream. Not only do the positive ions reduce the charging and collectioneificiency of the precipitator by neutralizing the negative ionsproducing negative charges on the dust particles, but the positive ionsproduce positively charged dust particles which tend to collect upon thedischarge electrode and thus interfere with maintenance of the properionizing field.

In a two-stage precipitator, back-corona is not ordinarily present inthe precipitating section, even though there is a heavy dust layerpresent, because of the almost complete absence of current between theelectrodes, as the only current is that carried by the dust particlesand usually is of the order of less than 1% of the current in theionizing field. However, there is often a very strong tendency towardback-corona in the is obtained. However, there are some circumstancesunder which humidification is definitely not practical, especially sincethe degree of humidification often runs from 50% to 95%. It may beundesirable under some circumstances to collect the material in dampform, and again the gas stream may be rendered exceedingly corrosive bythe presence of water vapor with the result "that replacement ormaintenance "costs on the precipitator are excessive unless corrosionresistant materials are used, and the cost of these materials may beprohibitive. Also, humidifying apparatus is expensive and requiresconstant attention to insure its proper functioning.

It is thus a general object of my invention to eliminate back-coronafrom the charging section of a follow-up or two-stage precipitator andat the same time to collect the suspended particles charging section ofa two-stage precipitator. It

has been found that in charging sections havingnon-dischargingelectrodes upon which dust particles accumulate, veryconsiderable trouble with back-corona arises when the dust is of aparticularly non-conducting nature. It will be appreciated that theamount of dust collected upon a non-discharging electrode in thecharging section is relatively small compared with the quantity ofdustcollected in the precipitating section, but that nevertheless .anamount which cannot be neglected always accumulates.

Since a given voltage will be required tobreak down a given layer ofdust, and since that voltage is numerically equal to the product of thecurrent density through the layer times the specific resistance of thelayer times the thickness of the layer, the tendency to produceback-corona may be considered as the product of these three factors. v

Reduction or elimination of back-corona in a charging field hasheretofore generally been accomplished by reducing the specificresistance of the material, and this has been done by humidification ofthe entire gas stream at a point well in advance of the precipitator. Ithas been found by experience that where the specific resistance of thematerial is less than approximately 10 ohms/cm. little or no troublewith back-corona is encountered. But for dusts having a specificresistance in excess of this approximate value, and which are referredto herein as those having a relatively high specific resistance orhaving relatively non-conducting characteristics, the tendency towardsback-corona becomes increasingly great with the increase in resistance.By adequate humidification of the gas stream, the material absorbsmoisture andthe average resistance of the layer can be kept below thisvalue, with the result that suitable collection emciency in asubstantially dry condition.

It is an object of my invention to provide electrical precipitationapparatus of the .twwstage type which maintains a relatively highcollection efficiency at all times because it is operated andconstructed so as to prevent the occurrence of back-corona dischargethat would reduce the normal collection efiiciency.

It is also an object of my invention to eliminate the necessity ofhumidification of the gas stream, since by eliminating humidificationthere are also eliminated various other problems and costs such as thedifficulties of handling a wet precipitate, thecost of thehumidification system, and the high costs of installation or maintenanceas a result of corrosion produced by the wet gases.

These objects of my invention are attained according to my improved andnovel method of operating electrical precipitation apparatus, by

' charging electrode in the first field with liquid to preventaccumulationpf particles on-the surface of the electrode and collectingthe particles in the second field in a substantially dry condition ondry collecting surfaces of the non-discharging electrodes. In atwo-stage precipitator constructed according to my invention andoperating according to the novel method above set forth, I provide, inaddition to the conventional elements mentioned above, means formaintaining a substantially continuous liquid film'only on thenon-discharging electrode or electrodes opposing the discharge electrodein the first or charging field, the film being exposed to the gas streamand flushing the electrode surface clean of any precipitated particles.In the collectingsection, I provide collecting electrodes with drysurfaces upon which the suspended particles are precipitated in orderthat the particles be collected and maintained in a substantially drycondition.

These novel improvements may be added to two-stage precipltators ofotherwise conventional construction, though it may be necessary in somecases to modify the electrodes to provide a precipitator constructedandoperated according to my invention. For purposes of fully disclosing myinvention and showing a typical embodiment thereof, I hereinafterdescribe my invention as applied to a. preoipitator of the typedescribed in my co-pending application mentioned above, in

which all electrodes extend transversely of the stream now, However, Iam not to be limited thereto, since my invention may also be applied toother types, as for example pipe or tubular types in which theelectrodes are parallel to the gas flow.

In a preferred form of my invention I provide a gas directing memberahead of each non-discharging electrode of the charging field to preventdirect impingement of the entering gas stream on the liquid filmmaintained over the surface of the non-discharging electrode. Thisgas-directing member also serves to direct the gas stream into the spaceoccupied by the discharge electrode, thus making for more efilcientcharging of the particles in suspension in the stream.

How the above objects and advantages of my invention, as well as othersnot specifically mentioned, are achieved will be more readily understoodby reference to the following description and the annexed drawings, inwhich:

Fig. 1 is a vertical median section through a each electrode l8, pipe I!carrying angularly spaced studs which center the pipe within theelectrode and maintain the electrode in a vertical position after inletpipe I9 is properly positioned. Each pipe 19 has a valve 2| whichregulates the flow of wash liquid into the electrode interior, and pipesl9 are connected to a header 22 which is connected to a main supply line23, as shown in Fig. 3. In order to facilitate assembly and inspectionof the parts, housing II is provided with a cover plate 24 over the topof compartment 1811, as the plate may be removed to permit header 22 andthe attached inlet pipes l9 to be lifted vertically upward out of theelectrodes. In operation, liquid from the interior of each electrode l6overflows the open top end and maintains on the exterior surface of theelectrode a substantially continuous film of liquid. Wall means l8shields the upper ends of the tubes from the gas stream so that thewater films are formed in a space isolated from the gas stream, anarrangement that favors formation two-stage precipitator constructed inaccordance with my invention;

Fig. 2 is a horizontal section taken on line 2-2 of Fig. 1;

Fig. 3 is an elevation looking from the left of Fig. 1, showing thesystem of electrodes in the charging field in elevation; and

Fig. .4 is a vertical transverse section taken on line 4-4 of Fig. 2.

From Fig. 1 it will be seen that the electric precipitator com-prisesgenerally three main parts, the ionizing or charging section-indicatedgenerally at ill at the entrance side of housing II, the collecting orprecipitating section indicated generally at l2 located to the right ofthe charging section and removed from the charging section in thedirection of gas flow (indicated, by the arrows), and the dustcollecting hopper" indicated generally at I 4 located below thecollecting section of the precipitator to receive the dust which fallsfrom the electrodes.

Charging section ll) of the preci'pitatorcontains the first system ofelectrodes that comprises in general one or more pairs of electrodesbetween which an ionizing field is established in which are charged theparticles suspended in the gas stream, each pair of electrodescomprising a discharge electrode and anon-discharging electrode. As willbe apparent from the subsequent description, ;the. system of electrodesshown in Figs. 1, 2 and 3 disclosing a preferred embodiment of myinvention comprises a plurality of spaced, parallel, non-dischargingelectrodes IS with a discharge electrode l5 spaced midwaybetween eachtwo non-discharging electrodes. Non-discharging electrodes 16 areyertically extending cylindrical members placed in a common planeextending transversely of housing II and the gas stream. Theseelectrodes are located adjacent the gas inlet and rest in sump ll in thebottom of the housing. The upper end of each electrode 16 passes withclearance through an opening Na in plate l8 which has a U-shaped portionthat forms a rectangular compartment lab at the top of the housing.

Electrodes l6 are preferably hollow tubes in order to provide aconvenient means for maintaining over their exterior surface, whichv isexposed to the gas stream, a substantially continuous film of water orother suitable wash liq uid. For this purpose a liquid inlet pipe I!exof an uninterrupted film. This liquid runs down the outside surface ofthe electrode, openings [8a being large enough for this purpose but notlarge enough to admit the gas stream to compartment I8b. Liquid collectsin sump l1, and is discharged from the precipitator through. Pipeconnection 25 to which a suitable drain-pipe, not shown, may beattached.'

Other means of forming water films are known in the art and may be usedinstead of the overflow arrangement shown. For example, a film may beformed by jets or sprays directed onto the upper end of the electrode,as may be seen from U. S. Patent 1,250,088 issued toBurns on Dec. 11,1917; or the electrode may be provided with suitable openings in thewalls through which liquid flows to reach the electrode surface in amanner shown by Schmidt Patent 1,309,221, issued July 8, 1919.

Non-discharging electrodes l6 are grounded through housing I1 and opposethe discharge electrodes I5 which are maintained at a relatively highpotential with respect to the grounded electrodes. Discharge electrodesI5 are mounted on an open rectangular frame 28, each discharge electrodebeing mounted between a pair of forwardly projecting arms 28a. and 28bwhich extend outwardly from the upper and lower sides of the framerespectively. All electrodesv I5 are preferably in a common'transversevertical plane. It is notnecessary that the discharge electrodes beexactly'in the vertical plane established by the axes of electrodes l6,but each discharge electrode is spaced equidistant from. the twoelectrodes [6 on either side. i

Electrode frame 28 is suspended from transverse cross-bar 30 and isattached thereto by a pair of hooks 3| which slide downwardly intosuitable openings in the crossbar. can be detached by lifting itvertically. Frame 28 is held in a vertical position and kept frompivoting around cross-bar 30 by engagement of an intermediate horizontalframe member 280 with a rigid depending stop member 30a. attached tocrossbar 30. Supporting bar 30 is electrically insulated from housing,I] and is supported at either end on an electrical insulator 33 which isenclosed withina separate insulator compartment 34 outside the wall ofhousing II in order to keep the insulator out of the main gas stream andas free as possible from any accumulation of dust Particles. v i

,A high potential diiference is maintained be The frame,

tween electrodes i5 and electrodes l6 by appli-.

cation of a high tension current to cross-bar 30 and the attachedelectrodes I5 through electrical cable 35 which is attached to asuitable source of high tension current, not shown. It is commonpractice to connect lead 30 to the negative side of a source ofunidirectional current; and since suitable power sources are well-knownin the art, it is not necessary to describe one here. Lead-in 35 entersinsulator compartment 34 through an insulator 36 and is connected to bar36 by strap 31.

A single pair of electrodes l 5 and i5 can maintain an ionizing fieldonly within a limited space, and consequently it may be necessary toprovide a plurality of pairs of these electrodes. The number of pairsprovided is determined by the size of the gas conduit formed by housingII, and is suflicient to maintain a substantially continuous chargingfield entirely across housing ll so that the entire stream of gaspassing through the housing is subjected to the action of this field inorder to charge the particles suspended in the gas stream.

Ahead of each non-discharging electrode i6 is placed a verticallyextending gas directing member 40, as may be seen most clearly from Fig.2. Bailles' 40 extend the full height of the gas passage and areattached at their upper ends to plate IS. The exact shape of thesemembers is not critical but it i preferred that the maximum horizontalwidth of each baille be greater than the diameter of the electrode I6with which it is aligned in the direction of gas flow. The gas directingmembers prevent the gas stream from directly impinging on the liquidfilm, which has two main advantages. Continuity of the film is moreeasily maintained since the force of the gas stream against the film islessened, and there is less tendency to precipitate dust particles onthe film since the volume of gas coming in direct contact with theelectrode is decreased. There is a further advantage in that the shapeof members 40 is such as to direct the gas stream toward or arounddischarge electrodes l5 and thus concentrate the gas stream wherecharging of the suspended particles is best effected.

Normally the parts described may be made of ordinary grades of steel oriron, as so little moisture is picked up by the gas stream that nocorrosion occurs; but should the liquid absorb sufficient gas to becomecorrosive, only the wetted parts need be made of corrosion resistantmetals. This is a great saving over prior practice in which the entireprecipitator and the ducts leading to and from it were subjected to anycorrosive action.

At a location in housing I I close to but spaced from the chargingsection in the direction of gas flow, is the second electrode systemcontained in the precipitating or collecting section of theprecipitator. This second system of electrodes comprises, in general,one or more pairs of nondischarging electrodes between which anelectrostatic field substantially free from corona discharge isestablished. In -the preferred embodiment of my invention hereindescribed, the second system comprises a plurality of pairs ofnondischarging electrodes. Each pair of electrodes comprises a groundedelectrode 43 and a high tension electrode 44 which is electricallyinsulated from the housing, these two electrodes having dry surfaces andbeing placed in parallel planes extending transversely across thehousing and the stream of gas flowing through the housing.

Grounded electrodes 43 are larger than electrodes 44 and, as may be seenfrom Fig. 4, substantially fill the entire cross-section of housing ll,except at the four corners. The construction of the grounded electrodesmay be seen in the left-hand half of Fig. 4 wherein one such electrodeis shown in elevation. Although it is to be understood that othertypes'of construction are suitable for the electrodes. the typedescribed is typical. The electrode comprises .a frame member 45 whichis preferably a hollow tube of suitable diameter that forms therectangular central portion of the electrode. Across this rectangularframe is placed a foraminous metal sheet 46 which is provided with asufficiently large number of gas openings to render the electrode easilyfluid-pervious. Although the metal sheet 46 may be formed in variousways, such as by woven-wire screening, it is preferred that theelectrode have a fiat surface such as provided by rolling expanded metalfiat or punching necessary openings in a flat metal sheet. Metal sheet46 extends over the entire portion of the electrode formed byrectangular frame member 45, and, as is seen from Fig. 4, this portionof the electrode extends out to the vertical side walls of housing il.

At the top and bottom of each electrode (except the end electrodes asdescribed later) is an impervious plate portion 41 and 48 respectively,

mounted on frame 45. Plate 41 extends upwardly from frame 45 intocontact with the top wall open bottom side of the housing. Theseimpervious portions of the electrode do not extend the full horizontalwidth of the housing, in order to provide electrical clearance forlongitudinally extending members located at the four corners of thehousing, as will be more fully described. At each vertical edge of theelectrode there is provided a marginal plate 50 which is imperforate andreduces the gas pervious portion of the electrode 43 to a size thatcorresponds approximately to the fluid pervious portion of the hightension electrodes 44. It is desired that the fluid pervious portions ofall transverse collecting electrodes be approximately the same in sizeand shape in order to reduce the tendency for the gas stream to by-passany of the electrodes.

Other electrode constructions and also other means of baiiling that maybe used to further reduce by-passing of the gas, are described in myco-pending application Serial No. 248,348, referred to above.

Each electrode 43 is held in a vertical plane across the gas stream bytwo pairs of pins 6| located in each housing side wall and engaging thevertical side of the electrode. Horizontal pins 52 in the electrodeframe 45 engage the upper set of pins 5| and support the weight of theelec- The materials used in the high tension elec- .trodes 44 are thesame as those used in the grounded electrodes 43 Just described. Thedetails of construction of an electrode 44. are illustrated in theright-hand half of Fig. 4 wherein one half of such an electrode is shownin elevation. Each electrode .44 has a rectangular frame of tubing 45aof approximately the same size and shape as the correspondingrectangular frame of electrode 43, except that the high tensionelectrodes are not as wide because they must be spaced from theside-walls of the housing in ings in longitudinally extending bus-bars55 located at the four corners of the housing. A transverse pin throughthe top of each extension of the side frame members supports the weightof the electrode from the upper two bus-bars 55, while the lower twobus-bars 55 engage the bottom of the electrode frames to keep theelectrode in vertical alignment and parallel to grounded electrodes 43.High tension electrodes 44 have no imperforate marginal portions 41, 48or 50 as described in connection with electrodes 43, the entireelectrode being fiuid-pervious and spaced around-its periphery by therequired electrical clearance from the grounded housing.

As may best be seen from Fig. 2, the two longitudinal bus-bars 55 at thetop of the housing are supported at their ends on two transverse bars 58which extend through the side-walls of the electrical insulators 51enclosed within separate insulator compartments 58 outside the housingwalls. In this way insulators 58 are removed from the main gas streamand protected from adeposit of dust. The lower two bus-bars 55 aresuspended from similar transverse members 88 which are supported attheir ends by depending insulators 8| (see Fig. 4) placed in separatelyenclosed insulator compartments 82.

A high potential from some suitable source, not shown, is applied to thehigh tension electrodes 44 by electrical lead-in cable85 which entersone jacent the top and-bottom margins respectively .of the firstelectrode 48 in order to prevent the .85 housing and are supported attheir ends'uprui" of the insulator compartments 58 through insulator 88.Electrical connection to supporting busbars 58 and 55 is completedthrough strap 81 attached to one of the transverse members 58. Sourcesof high potential are common knowledge in the art and need not bedescribed here, but it is common practice to apply to the high ten.-

sion electrodes a negative potential from a source 3 of unidirectionalrectified current.

Grounded electrodes l6 and 43 are connected electrically to housing IIto which a suitable ground connection is attached, as indicateddiagrammatically at 88 in Fig. 4. 4

In will be observed from Fig. lthat the firfi and last two of thegrounded collecting electrodes 48 are modified slightly from the othersin that these electrodes have no top or bottom bafiles'" and 48. Also,the rectangular frames45 of the second and next to the last electrodesare slightly smaller vertically than the othersin order to afford theproper electrical clearance between the electrodes and the transversemembers 58 and .68.

- the collecting electrodes.

Top and bottom baflies 41 and 48 are replaced at gas stream fromlay-passing the electrode. A similar construction is used at the outletend of the collecting section where baffles 12 and 18 are placed at thetop and bottom of the last electrode 48. These baflies are likewiseattached to the housing walls and extend into engagement with the lastelectrode.

Underneath collecting section II is located hopper l4 which hasdownwardly converging sidewalls that terminate at their lower ends at anopening into the upper side of the housing of screwconveyor 18. ConveyorI5 is driven by chain 18 from motor 11 and is designed forautomaticdischarge of the dust from the conveyor, since plate 18 closingthe discharge end of the conveyor housing isnormally held closed byspring I8 butcan be moved to the right against the spring by thepressure of material moved by the conveyor. Discharged dust falls downout of the right-hand end of the conveyor housing. Housing H is open onits under side throughout that portion of its length which is underneaththe collecting electrodes, and dust falling from these electrodes passesthrough this opening in the bottom of the housing and enters hopper H.In order to reduce or eliminate any tendency of the gas stream to flowdownwardly into thejropper and thus by-pass the collectingrelectrodes,vertically extending baflles amm suitable number are provided atintervals throughout the length of 'hopper l4. The'location, spacing andnumber of these is such as to accomplish the desired end of preventinggas flow through the hopper, but at the same time not to interfere withthe proper collection of dust.

Although the collecting electrodes are, as stated above, self-cleaningto a large extent, it may be desirable under some circumstances toeffect a more thorough cleaning of them, and for this purpose mechanicalcleaning means is provided. Such a mechanical cleaning .means may takeother forms than the one shown, but the manually actuated cleaning meansillustrated in Fig. 1 is simple to operate and construct. This cleaningmeans comprises a rotatable shaft 8| which extends horizontally the fulllength of hopper i4 and is journaled in bearings mounted on the endwalls of the hopper. The shaft is capable of limited axial movement inthese hearings. On one end of shaft 8| extending outside the hop.- peris placed handle 82. A plurality of hammers 83 are attached to shaft 8|and are all aligned in a single axial plane, each hammer comprising aweight on the. end of a rod of such length that, when the shaft 8| isrotated from the position shown in Fig. 1, the weights engage lowerframe members 45 and 45a of electrodes 43 and 44. The electrode cleaningmechanism normally occupies the position shownin Fig. 1 when itis not inoperation. To clean the electrodes, the operator grasps handle 82 androtates shaft 8| 9. half-tum, which places the weight ends of hammers 83in the spaces between v The operator thenreciprocates shaft 8|longitudinally, thus striking the lower frame members of the electrodeswith hammers 83 to jar loose accumulated deposits of precipitated dust.A few blows of this nature are 'suflicient to loosen the dry dust whichfalls through the bottom of housing 'into hopper l4, and after theoperation is completed, the hammers of their own weight return to theinoperative position shown in Fig. 1.

When it is desired to place the precipitator in operation. but beforethe gas stream is passed through the housing, valves 2| are open toadmit wash liquid to the interior of electrodes 16 and the electrodesare adjusted in position until the liquid overflows evenly around theentire upper rim of the electrode to form a thin film of liquid flowingdown the entire outside surface of the electrodes. It is desirable thatthis film be unbroken over the entire surface of each tube l6, since thegas stream swirls around the electrodes and comes in contact with theentire outside surface. gas is passed through housing II, and it firstcomes into engagement with gas directing members 40 which direct the gasstream away from the non-discharging electrodes I6 and toward thedischarge electrodes i5. This concentration of the gas around thedischarge electrodes facilitates charging of all suspended particles.Bailles 40 prevent the gas stream from directly impinging upon theliquid films maintained over the exterior surface of each electrode i6,thus keeping the film continuous at all times and preventing breaks inthe film caused by the gas stream striking the film. Also, there is lesstendency for particles to precipitate upon the electrodes l6, since asmaller portion of the gas stream comes into contact with the electrodesthan when members 40 are omitted.

It is inevitable that there will be some precipitation of dust particlesupon the non-discharging electrodes of the first electrode system, sincethe gas stream cannot be kept out of any contact with the electrodes andno matter how short the time in which the gas stream is exposed to thecharg- After the liquid films are established,

ing field, that time is still of finite duration and sufficient to causea small amount of precipitation. However, the precipitation is now uponthe film rather than upon the dry metal surface of the electrode itself,and all the precipitated particles are immediately carried away by theflushing action of the liquid film with the result that there is noinsulating layer of poorly conducting material built up on the exteriorof the non-discharging electrode. In this manner, the condi tionsessential to the formation of back-corona are prevented from occurringand back-corona from this source is completely eliminated in thecharging field.

Experience has shown that there is no material increase in humidity ofthe gas stream, be-

cause the'time of exposure to the liquid is so short that very littleevaporation occurs.

As the gas stream passes the first system of electrodes, the suspendedparticles pass through the harging field maintained across the entirewidth of the housing between the pairs of electrodes l5 and I6. Assumingstandard practice is followed in supplying unidirectional current to thehigh tension electrodes, the discharge electrodes ii are of negativepolarity and conse quently the particles suspended in the gas streambecome charged negatively. These dry charged particles are then carriedby the gas stream toward the first transverse collecting electrode whichmay be a high tension electrode 44 but which is here shown as a roundedelectrode 43 which is consequently oppositely charged or of positivepolarity with respect to the suspended particles. Some of the negativelycharged particles are attracted to the positively charged electrode 43and are precipitated on the front or upstream side thereof in a drycondition, these precipitated particles becoming neutralized or evenpositively charged.

The remaining negatively charged particles pass through the gas openingsin the first electrode and enter the precipitating field maintainedbetween the first two collecting electrodes. This field is in adirection to force the charged particles toward the grounded electrodeagainst the movement of the gas stream, and consequentl part of theremaining particles are precipitated in a dry condition on the rear ordownstream face of the first electrode 43. This deposition'is alsofacilitated by eddying of the gas stream as it passes through theelectrode, since the eddies .tend to carry dust particles toward theback face. Negatively charged particles as yet unprecipitated passthrough the first high tension electrode 44 and enter the secondprecipitating field, oppositely directed tothe first, that tends toprecipitate particles on the front face of the second grounded electrode43. This action is'repeated at each electrode 43, and the negativeparticles are subjected to a series of successively oppositely directedfields that eiTect substantially complete initial precipitation in a drycondition on the first few grounded electrodes.

If for any reason there are positively charged particles in the gasstream, these particles tend to go through the first electrode andprecipitate upon the first high tension electrode 44, since the hightension electrode is negatively charged and is opposite in sign to thepositively charged particles.

The collecting section is preferably composed of transverse fluidpervious electrodes as described, the advantages of this constructionbeing more fully discussed in my co-pending application referred toabove. However, my invention is not limited thereto, as any suitablecollecting electrodes with dry surfaces may be used, as, for example,plates parallel to the gas flow as illustrated by Schmidt Patent1,343,285. Likewise, other changes in arrangement and construction ofparts may be made without departing from the spirit and scope of myinvention. Consequently, the foregoing description is to be consideredas illustrative of a typical embodiment of my invention rather thanlimitative thereon,

and the appended claims are to be construed accordingly. I claim:

1. The method of removing suspended particles from a gas stream whichcomprises subjecting the gas stream to the action of corona discharge ina high tension electric field maintained between a pair of electrodes ofwhich one electrode is substantially non-discharging, and in which fieldthe suspended particles are charged and some of them are removed fromthe gas stream; and subsequently passing the gas and the remainingsuspended particles through a second high tension electric fieldsubstantially free from corona discharge; maintaining in said firstfield a subs'tantially continuous film of liquid over the nondischargingelectrode; and collecting charged particles in said second field in asubstantially dry condition.

2. The improvement in the method of removin'g substantiallynon-conducting suspended particles from a stream of gas by passingthegas stream through successive electric fields in the first of which theparticles are charged by corona discharge and some are precipitated andin the second of which particles charged in the first field areprecipitated in an electric field substantially free from coronadischarge, which comprises continually fiushing the surface of anondischarging electrode in the first field to prevent accumulation ofparticles on the electrode, and collecting particles in the second fieldin a substantially dry condition.

3. In an electrical precipitation apparatus through which moves a streamof gas containing suspended particles to be removed, the combination ofa first complementary electrode system comprising a non-dischargingelectrode and a discharge electrode adapted to create an ionizing field;a second complementary electrode system comprising spacednon-discharging electrodes with dry surfaces; means for directing a gasstream to be treated successively between the complementary members ofsaid electrode systems; means for impressing a high potential across thecomplementary electrodes of each of said electrode systems; and meansfor maintaining a liquid film only on the non-discharging electrode ofthe first system.

4. Electrical precipitation apparatus as in claim 3 in which a gasdirecting member is placed ahead of the non-discharging electrode of thefirst system to prevent direct impingement of the entering gas streamagainst the liquid film.

5. In an electrical precipitation apparatus through which moves a streamof gas containing suspended particles to be removed, the combination ofa first pair of electrodes of which one is a non-discharging electrodeand the other is a discharge electrode; means for maintaining asubstantially continuous liquid film on said nondischarging electrode ofsaid first pair; means for maintaining a high potential differencebetween said electrodes to establish a charging field; a plurality ofnon-discharging collecting electrodes having dry surfaces disposedbeyond said first pair of electrodes in the direction of stream flow,each of said non-discharging collecting electrodes being fluid-perviousand extending transversely to the direction of stream flow; and means ior applying a potential difference between successive ones of thenon-discharging collecting electrodes to establish a plurality ofsuccessive precipitating fields in which suspended particles arecollected in a substantially dry condition.

6. In an electrical precipitation apparatus through which flows a streamof gas containing suspended particles to be removed, the combination ofa first electrode system disposed transversely to the gas stream andcomprising a finewire discharge electrode spaced between a pair ofopposing non-discharging electrodes; means for maintaining asubstantially continuous liquid film on the outside surfaces of saidnon-discharging electrodes of the first system; means for applying ahigh potential difference between the electrodes of said first system toestablish a charging field; a second electrode system comprising aplurality of non-discharging collecting electrodes having dry surfacesdisposed beyond said first system of electrodes in the direction ofstream fiow, each of said non-discharging collecting electrodes beingfiuid pervious and extending transversely in the direction of streamfiow; and means for applying a potential difference between successiveones of the non-discharging collecting electrodes to establish aplurality of successive precipitating fields in which suspendedparticles are collected in a substantially dry condition.

7. Electrical precipitation apparatus as in claim 6 in which a gasdirecting member is placed ahead of each non-discharging electrode ofthe first system to prevent direct impingement of the entering gasstream against the liquid film on that non-discharging electrode.

8. In electrical precipitation apparatus for cleaning a gas stream ofthe separated field type having a first pair of electrodes between whichan ionizing field is maintained and a second pair of electrodes betweenwhich a non-discharging precipitating field is maintained, thecombination comprising: a non-discharging electrode in the ionizingfield; means for maintaining a substantially continuous liquid film uponthe surface of said non-discharging electrode exposed to the gas streamas the gas stream moves through the ionizing field; and an electrode inthe precipitating field of the same polarity as said non-dischargingelectrode and having a dry surface upon which the particles arecollected in a substantially dry condition.

9. In an electrical precipitation apparatus through which fiowshorizontally a stream of gas containing suspended particles to beremoved, the combination of wall means forming a compartment isolatedfrom the gas stream; a plurality of vertically extending non-dischargingelectrodes spaced transversely of the gas stream, the upper ends of theelectrodes extending with clearance through the wall means into thecompartment; means at the upper ends of the electrodes for formingand-maintaining a liquid film over the exterior surface of eachelectrode, each film being formed inside said compartment away from thegas stream; andmeans at the lower ends of the electrodes for collectingand discharging the liquid from the precipitator.

10. Electrical precipitation apparatus as in claim 3 in which thenon-discharging electrode of the first system is a substantiallycylindrical member extending vertically and transversely of the gasstream, and in which a gas directing member extending substantially thefull height of said non-discharging electrode of the first system isaligned therewith and shields the electrode and liquid film thereonagainst direct impingement of the entering gas stream.

' HARRY J. WHITE.

